In long distance telecommunication, received carrier levels are amplified to usable values with an AGC amplifier. It is desirable to linearize the AGC voltage (which controls the AGC amplifier) such that it is a linear function of the received carrier level. Various techniques have been used to accomplish this linearization, and typically involve auxiliary circuitry having a nonlinear response for compensating the existing nonlinear signal response curve.
One type of scheme is generally referred to as a successive approximation method. This method involves the use of a plurality of diodes which variously turn on at their correspondent threshold levels as the nonlinear input rises. The more diodes which turn on, the greater current is drawn. Though the nonlinear input is a continuously variable curve, the compensating adjustments are in a step-wise fashion because of the diodes, and hence this method is sometimes referred to as the piece-wise linear method because the diodes create a transfer characteristic of a series of connected straight lines of varying slope. Accuracy can be improved by adding more diodes, but this increases cost and complexity.
Another scheme is the logarithmic approximation method. This method makes use of the fact that the input signal response curve to be linearized is approximately logarithmic, and that older type low voltage zener diodes also exhibit this characteristic in the vicinity of their reverse threshold breakover, or knee. A regular diode, although logarithmic, has a very small usable voltage differential, so small as to make it unusable for this type of application. An older type zener diode, however, biased on the knee of its response curve, has a much longer usable region because of what is known as a sloppy knee wherein the reverse breakover is not sharp but rather more gradual and including a logarithmic type curvature portion. A disadvantage of this technique is the noise encountered which is inherent in zener diodes. Another disadvantage of this technique is the limited sourcing availability. Almost all zener diodes are now made with improved characteristics providing a very sharp reverse breakover threshold or knee which is not suited to the above linearization technique.